Combustion analyzers are used to determine the concentration of one or more components of a sample, by combusting the sample and analysing the gaseous products for specific oxides. Typically, the carbon, sulphur and/or nitrogen content of the sample is measured by detecting CO2, SO2 and NO, respectively.
A schematic illustration of a typical combustion analyzer is shown in FIG. 1. The combustion analyzer 10 comprises a sample introduction stage 20, a combustion stage 30, a conditioning stage 40, and a detection stage 50. The sample introduction stage 20 comprises a sample introduction apparatus 22, to which are connected a supply of a sample 24, a supply of oxygen 26 and a supply of argon 27. The sample introduction apparatus 22 introduces these fluids into a combustion chamber 32 in a suitable form for combustion to take place. A further supply of oxygen 25 may be provided, directly into the combustion chamber 32. The combustion chamber 32 is heated by an electric heater 34, so that the sample is delivered into an oxygen-rich atmosphere at high temperature, typically of around 1000° C. The sample is thereby converted into various combustion products, such as CO2, H2O, SO2, NO, etc. The combustion products leave the combustion chamber 32 and pass through the conditioning stage 40, where processes such as cooling, filtering, drying, etc. take place. The conditioned products then pass through one or more dedicated detectors 52, 54, in which properties of the components of the combustion products may be detected. For example, CO2 may be detected by absorption of infrared radiation, using a non-dispersive infrared (NDIR) detector; SO2 may be detected by fluorescence with ultraviolet light, using a light sensor; and NO can be detected from de-excitation processes following its reaction with ozone (O3) to form excited NO2, using a chemiluminescence light sensor. The detected signals are indicative of the respective amount of each component of the combustion products and can therefore be related to the composition of the original sample. Finally, the detected combustion products are passed out of the detection stage 50, as waste products 56.
The performance of such a combustion analyzer 10—in terms of its suitability, reliability, accuracy and robustness—depends strongly on its ability to convert the element(s) of interest in a sample into its/their respective oxide(s).
For combustion analysis of a sample containing sulphur, the combustion product to be detected is sulphur dioxide (SO2). The achievable yield of SO2 which may be detected with current combustion analyzers is around 90%. The yield is the proportion of the amount of sulphur originally contained in the sample which is actually converted to sulphur dioxide. The achievable yield of a combustion analyzer is calculated by analysing known, standard samples for calibration purposes. Once a calibration curve has been measured using standard samples, unknown samples may be analyzed and the detected values may be calibrated accordingly. However, samples and also combustion conditions in a combustion analyzer are subject to variation, with the result that the calibration curve cannot consistently provide accurate measurements from sample to sample.
Also, current compliance regulations for sulphur in petrochemical fuels mean that total sulphur specifications (i.e., the permissible amount of sulphur in any form) are at low parts per million (ppm) levels and are heading ever lower, towards sub-ppm levels. For example, diesel specifications for sulphur are soon expected to be 10 ppm in the EU and 15 ppm in the US; for gasoline (petrol), the specifications are expected to be 10 ppm in the EU and 80 ppm in the US. It is therefore increasingly important to be able to measure sulphur concentrations at such low levels.
Accordingly, it would be desirable to provide an improved apparatus and method for combustion analysis of samples containing sulphur.